Resistor with temperature coefficient of resistance (TCR) compensation

ABSTRACT

A current sense resistor and a method of manufacturing a current sensing resistor with temperature coefficient of resistance (TCR) compensation are disclosed. The resistor has a resistive strip disposed between two conductive strips. A pair of main terminals and a pair of voltage sense terminals are formed in the conductive strips. A pair of rough TCR calibration slots is located between the main terminals and the voltage sense terminals, each of the rough TCR calibration slots have a depth selected to obtain a negative starting TCR value observed at the voltage sense terminals. A fine TCR calibration slot is formed between the pair of voltage sense terminals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/218,219, filed Jul. 25, 2016, issuing as U.S. Pat. No. 9,779,860 onOct. 3, 2017, which is a continuation of U.S. patent application Ser.No. 14/531,505, filed Nov. 3, 2014, issued as U.S. Pat. No. 9,400,294 onJul. 26, 2016, which is a continuation of U.S. patent application Ser.No. 14/015,488, filed Aug. 30, 2013, issued as U.S. Pat. No. 8,878,643on Nov. 4, 2014, which is a continuation of U.S. patent application Ser.No. 13/493,402, filed Jun. 11, 2012, issued as U.S. Pat. No. 8,525,637on Sep. 3, 2013, which is a continuation of U.S. patent application Ser.No. 12/874,514, filed Sep. 2, 2010, issued as U.S. Pat. No. 8,198,977 onJun. 12, 2012, which claims the benefit of U.S. Provisional ApplicationNo. 61/359,000, filed Jun. 28, 2010, and U.S. Provisional ApplicationNo. 61/239,962, filed Sep. 4, 2009, the entire contents of all of whichare hereby incorporated by reference as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to a four terminal current sense resistorof very low ohmic value and high stability.

BACKGROUND

Surface mounted current sense resistors have been available for theelectronic market for many years. Their construction typically includesa flat strip of a resistive material that is coupled between highconductivity metal terminals forming the main terminals of the device. Apair of voltage sense terminals can be formed in the main terminalsthereby creating a four terminal device. The main terminals carry themajority of the current through the device. The voltage sense terminalsproduce a voltage that is proportional to the current passing throughthe device. Such devices provide a mechanism to monitor the currentpassing through a given circuit using conventional voltage sensingtechniques. The actual current passing through the device can bedetermined based on the sensed voltage and the resistance value of thedevice as dictated by ohms law. An ideal device would have a TemperatureCoefficient of Resistance (TCR) that is close to zero. However, mostdevices have a non-zero TCR that can lead to inaccurate voltage readingsat the voltage sense terminals particularly when the temperature of thedevice varies.

In low ohmic current sense resistors and high current shunts, theresistive element length is short while the length of the resistor is astandard length, or in the case of high current shunts long because ofthe application. The long resistor length and short resistive elementlength causes a significant amount of copper termination metal to be inthe current path. Copper has a TCR of 3900 ppm/° C. while the resistivematerial is typically less than 100 ppm/° C. The added copper in thecurrent path drives the overall TCR of the resistor to values that canbe in the 800 ppm/° C. range or greater, versus a desired TCR of lessthan 100 ppm/° C.

As noted above, typical current sense resistors have four terminals, twomain terminals and two voltage sense terminals, separated by two slots.The length of two slots is manipulated to adjust TCR. See U.S. Pat. No.5,999,085 (Szwarc). This method does not lend itself to conventionalresistor calibration equipment such as a laser or other cuttingtechniques that are typically used to reduce the width of the resistiveelement to increase the resistor's resistance value.

What is needed is an improved configuration and method of making acurrent sense resistor with TCR compensation or adjustment. It wouldalso be desirable to provide an improved resistor configuration andmethod that simplifies TCR adjustment of current sense resistor duringthe manufacturing process. One or more of these aspects will becomeapparent from the specification and claims that follow.

SUMMARY

A resistor and a method of manufacturing a resistor with temperaturecoefficient of resistance (TCR) compensation are disclosed. The resistorhas a resistive strip disposed between two conductive strips. A pair ofmain terminals and a pair of voltage sense terminals are formed in theconductive strips. A pair of rough TCR calibration slots is locatedbetween the main terminals and the voltage sense terminals, each of therough TCR calibration slots have a depth selected to obtain a negativestarting TCR value observed at the voltage sense terminals. A fine TCRcalibration slot is formed between the pair of voltage sense terminals.The fine TCR calibration slot has a depth selected to obtain a TCR valueobserved at the voltage sense terminals that approaches zero. Theresistor can also have a resistance calibration slot located between thepair of main terminals. The resistance calibration slot has a depthselected to calibrate a resistance value of the resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a four terminal resistor with a pair of first slotsconfigured to adjust TCR to a negative starting value;

FIG. 2 illustrates a four terminal resistor with a pair of first slotsand a second slot configured to collectively adjust TCR to a minimumvalue;

FIG. 3 illustrates a four terminal resistor with a pair of first slotsand a second slot configured to collectively adjust TCR to a minimumvalue and a third slot configured for resistance calibration;

FIG. 4 is a graph showing the relationship between the second slot depthand TCR and resistance value;

FIG. 5 illustrates another embodiment of a four terminal resistor withTCR compensation; and

FIG. 6 is a graph showing the TCR compensation associated with thevarious slot formations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-3 show exemplary resistor geometries through various stages ofadjustment of the Temperature Coefficient of Resistance (TCR). It isunderstood that the techniques disclosed herein could also apply toother resistor types including film resistors, metal foil resistors andother types of resistor technologies.

FIG. 1, shows a resistor 10 generally formed of a resistive strip 13disposed between two conductive strips 12, 14. The resistor 10 has mainterminals 16, 18 and voltage sense terminals 20, 22. In operation, themain terminals 16, 18 carry the majority of the current passing throughthe resistor. A pair of first slots 24, 26 is located between the mainterminals and the voltage sense terminals. First slots 24, 26 each havean associated depth that extends towards the resistive strip 13. This isshown generally as depth A. It is understood that each first slot 24, 26can use the same depth A, or in the alternative, first slots 24 and 26can have different depths. FIGS. 2 and 3 show the formation of a secondslot having a depth B and a third slot having a depth C. Therelationship between these slots will be discussed below.

Returning to FIG. 1, the conductive strips are generally formed ofcopper sheet material and have a thickness typically in the range ofabout 0.008-0.120 inches (˜0.2-3 mm). The thickness of the copper isgenerally selected based on the desired power dissipation of the deviceand the desired mechanical strength (e.g., so that the resistor hassufficient strength during manufacture, installation and use).

The pair of first slots 24, 26 partition off a portion of the conductivestrips 12, 14 and create a four terminal device. The size and locationof the pair of first slots 24, 26 generally define the dimensions of themain terminals 16, 18 and the voltage sense terminals 20, 22. The pairof first slots 24, 26 is generally located towards one edge of theresistor. In this example, the pair of first slots 24, 26 are located adistance Y measured from the upper edge of the device. The Y distance isgenerally selected to yield appropriately sized voltage sense terminals.For example, the Y distance can be selected to provide voltage senseterminals of sufficient width to withstand punching or machiningoperations during manufacture and to have sufficient strength duringinstallation and use.

The first slots 24, 26 each have a depth generally shown as distance Ain FIG. 1. In most applications first slots 24, 26 will have the samedepth A. It is understood that first slots 24 and 26 could each beassociated with a different depth. It is also understood that the depthassociated with first slots 24, 26 could be referenced from a variety ofpoints on the device. Generally, the pair of first slots 24, 26 definesa reduced thickness or neck between the main terminals 16, 18 and thevoltage sense terminals 20, 22. This is shown generally as the distanceX in FIG. 1. A description of how the first slot depth A is determinedis set out below.

In the following example, conductive strips 12, 14 are formed of copper.As noted above, copper has a TCR of 3900 ppm/° C. In contrast, theresistive strip 13 may have a TCR of less than 100 ppm/° C. In absenceof the pair of first slots 24, 26, the resistor 10 would typically havea very high, positive TCR due to the large amount of copper disposed inthe current path. It is generally desirable to minimize the TCR (i.e., aTCR having an absolute value approaching zero). A typical range for agiven current sense resistor may be ±25 ppm/° C. Assume for this examplethat a given device has a target resistance value of 200 μΩ (i.e.,0.0002Ω). Also assume that the initial design without the pair of firstslots 24, 26 yields a device with a TCR of approximately 800 ppm/° C.

The thickness of the copper conductive strips 12, 14 is selected asdiscussed above. The dimensions of the resistive strip 13 are selectedto yield a resistance that is close to but below the target resistancevalue. This is done because the final resistance value will be set by asubsequent trimming operation (which will increase the resistance valueof the resistor).

Aside from defining the dimensions of the voltage sense terminals, thepair of first slots 24, 26 causes the TCR at the voltage sense terminals20, 22 to become more negative. The deeper the pair of first slots 24,26, the more negative the TCR at the voltage sense terminals 20, 22becomes. The pair of first slots 24, 26 does not significantly alter theTCR of the resistor itself, rather the pair of first slots 24, 26 alterthe TCR observed at the voltage sense terminals 20, 22.

Typically, the relationship between the first slot depth A, and the TCRobserved at the voltage sense terminals 20, 22 is determined via aprototyping process. For example, a prototype device is manufactured andthen tested using conventional methods (i.e., the voltage, current andtemperature is measured through a range of conditions). The depth of thefirst slots 24, 26 is successively increased until a negative startingTCR value is observed at the voltage sense terminals 20, 22, for exampleapproximately −200 ppm/° C. Thus, first and second slots 24, 26 can bethought of as rough TCR calibration slots.

A negative starting TCR value is desirable at this stage because asecond slot will be used to fine tune the TCR value as discussed in moredetail below. Once the proper first slot depth is determined, this depthis not altered for a particular style of product (i.e., resistors havingthe same physical and electrical characteristics). This is advantageoussince the pair of first slots 24, 26 can be inserted early in themanufacturing process using conventional punching, end milling or othermachining techniques. Subsequent slotting operations can be then carriedout later in the manufacturing process and can even be accomplished vialaser trimming.

Turning to FIG. 2, a second slot 28 having a depth B is shown locatedbetween the voltage sense terminals 20, 22. In general, the second slot28 is formed in the resistive strip 13 between the voltage senseterminals 20, 22. It is understood that the second slot can also resultin the removal of a portion of the voltage sense terminals 20, 22, asshown in FIG. 2. The net effect of the second slot 28 is to drive theTCR observed at the voltage sense terminals 20, 22 positive. The secondslot 28 will also cause a small increase in resistance value. This isshown graphically in FIG. 4. In this example, the TCR of the resistorwithout a second slot 28 (e.g., as shown in FIG. 1) is −198 ppm/° C. Theinitial resistance of the device (without second slot 28) isapproximately 110 μΩ (i.e., 0.00011Ω). With the second slot depth set to0.040″ (˜1 mm) the TCR improves to −100 ppm/° C. Similarly, theresistance increases to approximately 125 μΩ (i.e., 0.000125Ω).

Turing to FIG. 3, with the second slot 28 set to at 0.080″ (˜2 mm) theTCR continues to become more positive and approaches zero. Theresistance increases to approximately 140 μΩ (i.e., 0.00014Ω). Thus, thesecond slot 28 functions as a fine TCR calibration slot. As noted above,a typical target range for TCR range for a given device would beapproximately ±25 ppm/° C. The second slot 28 can be formed using lasertrimming techniques, conventional punching, end milling or any othermachining technique that will permit removal of material to a desireddepth and width.

FIG. 3 also shows a third slot 30 (resistance calibration slot) formedbetween main terminals 16, 18. The third slot 30 has a depth that isselected to fine tune the resistor value. In this case the depth C isselected to yield a target resistance value within specified tolerance(e.g., 200 μΩ±1%). The third slot 30 can be formed using laser trimmingtechniques, conventional punching, end milling or any other machiningtechnique techniques that will permit removal of material to a desireddepth and width.

It is understood that the first slots 24, 26 and the second slot 28 canbe formed at the same time or at separate times. It is also understoodthat the second slot 28 can be changed “on the fly” (e.g., if TCR ismeasured on a resistor by resistor basis). Thus, the TCR of eachresistor could be customized to a specified value. As an addedadvantage, the second slot 28 can be formed using laser trimmingtechniques which can greatly simplify the TCR adjustment process. Firstslots 24, 26 and second slot 28 shown in FIGS. 1 and 2 have a generallyrectangular profile. Third slot 30 shown in FIG. 3 has a generallytriangular profile. It should be understood that other simple or complexgeometric slot profiles could be used without departing from the scopeof this disclosure.

FIG. 5 shows another slot configuration for TCR compensation. FIG. 5shows a resistor 100 generally formed of a resistive strip 113 disposedbetween two conductive strips 112, 114. The conductive strips aregenerally formed of copper sheet material and have a thickness typicallyin the range of about 0.008-0.120 inches (˜0.2-3 mm). The thickness ofthe copper is generally selected based on the desired power dissipationof the device and the desired mechanical strength (e.g., so that theresistor has sufficient strength during manufacture, installation anduse).

The resistor 100 has main terminals 116, 118 and voltage sense terminals120, 122. In operation, the main terminals 116, 118 carry the majorityof the current passing through the resistor. The main terminals areformed with a defined internal area (e.g., spaced away from the edges ofthe conductive strips 112, 114). A pair of first slots 124, 126 islocated between the main terminals and the voltage sense terminals. Inthis embodiment the voltage sense terminals are formed within thedefined internal area of the main terminals. This configuration isdesirable for applications requiring more compact and centrally locatedvoltage sense terminals. First slots 124, 126 are formed with two legs.First leg 123 has a length that extends generally orthogonal to the maincurrent path as shown by “A.” Second leg 125 has a length extendsgenerally parallel to the main current path as shown by “B.” It isunderstood that first slots 124 and 126 can use the same leg lengths Aand B. In the alternative, first slots can have different leg lengths.The resistor 100 also has a second slot 128 having a depth C. Therelationship between these slots will be discussed below.

The pair of first slots 124, 126 partition off an internal portion ofthe conductive strips 112, 114 and create a four terminal device. Thesize and location of the pair of first slots 124, 126 generally definethe dimensions of the voltage sense terminals 120, 122. In this example,the sense terminals are located generally in the junction between thefirst and second legs 123, 125.

As discussed above, the first leg 123 has a length A and the second leg125 has a length B. FIG. 6 is a graph showing the TCR compensationassociated with the formation of the first slots 124, 126. Sample 1 is abaseline resistor configured without first slots 124, 126. In thisconfiguration, the TCR is +60 ppm/° C. Samples 2 and 3 show TCRcompensation as the first legs 123 are added (Sample 2) and increased inlength (Sample 3). As shown on the graph, the TCR becomes more negativeending at +20 ppm/° C. Samples 4 and 5 show TCR compensation as thesecond legs 125 are added (Sample 4) and increased in length (Sample 5).First legs 123 remain constant in samples 4 and 5. As shown on thegraph, the TCR becomes more negative ending at approximately −35 ppm/°C.

During manufacturing, the first leg 123 can be inserted first until arough level of TCR compensation is achieved. First legs can be formed bya variety of methods including punching or machining. The second leg 125can be then inserted to fine tune the TCR compensation to the desiredlevel, Second legs can be formed by a variety of methods including lasertrimming. In most applications first slots 124, 126 will have the samedimensions. It is understood that first slots 124 and 126 could each beassociated with other leg configurations. Once the first slots 124 and126 are completed, second slot 128 can be formed to fine tune theresistance value. First slots 124, 126 and first and second legs 123,125 as shown in FIG. 5 have a generally rectangular profile. Second slot125 shown in FIG. 5 has a generally rounded profile. It should beunderstood that other simple or complex geometric slot or leg profilescould be used without departing from the scope of this disclosure.

Based on the foregoing it is readily apparent that a variety ofmodifications are possible without departing from the scope of theinvention. For example the first slots 24, 26, 124, 126 can have variedspacing and depths. Similarly, variations in the location of the otherslots and the shape of the various terminals are possible. Those skilledin the art will recognize that a wide variety of modifications,alterations, and combinations can be made with respect to the abovedescribed embodiments without departing from the spirit and scope of theinvention, and that such modifications, alterations, and combinationsare to be viewed as being within the ambit of the inventive concept. Itis intended in the appended claims to cover all those changes andmodifications which fall within the true spirit and scope of the presentinvention.

What is claimed is:
 1. A resistor with temperature coefficient ofresistance (TCR) compensation, the resistor comprising: a resistivestrip disposed between a first conductive strip and a second conductivestrip; a first voltage sense terminal and a first main terminal formedin the first conductive strip separated by a first slot at a first sideedge of the first conductive strip; a second voltage sense terminal anda second main terminal formed in the second conductive strip separatedby a second slot at a second side edge of the second conductive strip,the first side edge of the first conductive strip and the second sideedge of the second conductive strip being on opposite ends of theresistor; the first voltage sense terminal and the first main terminalformed contiguously in the first conductive strip, and the secondvoltage sense terminal and the second main terminal formed contiguouslyin the second conductive strip; the resistive strip comprising a thirdslot positioned between the first and second voltage sense terminals, adepth of the third slot configured to adjust a TCR value of theresistor; and, the resistive strip comprising a fourth slot positionedbetween the first and second main terminals, a depth of the fourth slotconfigured to adjust a resistance value of the resistor.
 2. The resistorof claim 1, wherein the first slot and the second slot each have a depththat extends toward the resistive strip.
 3. The resistor of claim 2,wherein the first slot has an opening at the first side of the resistorfacing away from the resistive strip, and the second slot has an openingat the second side of the resistor facing away from the resistive strip.4. The resistor of claim 1, wherein the fourth slot is formed at an edgeof the resistive strip.
 5. The resistor of claim 1, wherein the thirdslot does not meet either the first slot or the second slot.
 6. Theresistor of claim 1, wherein a depth of the first slot and the secondslot are configured to adjust a TCR value of the resistor.
 7. Theresistor of claim 1, wherein each of the first and second slots has adepth selected to obtain a negative starting TCR value observed at thefirst and second voltage sense terminals, and wherein the third slot hasa depth selected to obtain a TCR value observed at the first and secondvoltage sense terminals that approaches zero.
 8. The resistor of claim1, wherein at least a portion of the third slot extends only between thefirst voltage sense terminal and the second voltage sense terminal.
 9. Amethod of manufacturing a resistor with temperature coefficient ofresistance (TCR) compensation, the method comprising: disposing aresistive strip between a first conductive strip and a second conductivestrip; forming a first slot at a first side of the first conductivestrip to form a first voltage sense terminal and a first main terminalin the first conductive strip; forming a second slot at a second side ofthe second conductive strip to form a second voltage sense terminal anda second main terminal in the second conductive strip, the first side ofthe first conductive strip and the second side of the second conductivestrip being on opposite sides of the resistor; forming a third slot inthe resistive strip between the first and second voltage senseterminals, the third slot having a depth selected to adjust a TCR valueof the resistor; and, forming a fourth slot in the resistive stripbetween the first and second main terminals, the fourth slot having adepth selected to adjust a resistance value of the resistor; wherein thefirst voltage sense terminal and the first main terminal are formedcontiguously in the first conductive strip, and wherein the secondvoltage sense terminal and the second main terminal are formedcontiguously in the second conductive strip.
 10. The method of claim 9,wherein the first slot and the second slot are each formed having adepth that extends toward the resistive strip.
 11. The method of claim10, wherein the first slot is formed having an opening at the first sideof the first conductive strip facing away from the resistive strip, andthe second slot is formed having an opening at the second side of thesecond conductive strip facing away from the resistive strip.
 12. Themethod of claim 9, wherein the fourth slot is formed at an edge of theresistive strip.
 13. The method of claim 9, wherein the third slot isformed such that it does not meet either the first slot or the secondslot.
 14. The method of claim 9, wherein a depth of the first slot andthe second slot are selected to adjust a TCR value of the resistor. 15.The method of claim 9, wherein each of the first and second slots has adepth selected to obtain a negative starting TCR value observed at thefirst and second voltage sense terminals, and wherein the third slot hasa depth selected to obtain a TCR value observed at the first and secondvoltage sense terminals that approaches zero.
 16. The method of claim 9,wherein the third slot is formed such that at least a portion of thethird slot extends only between the first voltage sense terminal and thesecond voltage sense terminal.
 17. A resistor with temperaturecoefficient of resistance (TCR) compensation, the resistor comprising: aresistive strip disposed between a first conductive strip and a secondconductive strip; a first voltage sense terminal and a first mainterminal formed in the first conductive strip separated by a first slotat a first side edge of the first conductive strip; a second voltagesense terminal and a second main terminal formed in the secondconductive strip separated by a second slot at a second side edge of thesecond conductive strip, the first side edge of the first conductivestrip and the second side edge of the second conductive strip being onopposite ends of the resistor; the resistive strip comprising a thirdslot positioned between the first and second voltage sense terminals, adepth of the third slot configured to adjust a TCR value of theresistor, wherein the third slot does not meet either the first slot orthe second slot; and, the resistive strip comprising a fourth slotpositioned between the first and second main terminals, a depth of thefourth slot configured to adjust a resistance value of the resistor. 18.A resistor with temperature coefficient of resistance (TCR)compensation, the resistor comprising: a resistive strip disposedbetween a first conductive strip and a second conductive strip; a firstvoltage sense terminal and a first main terminal formed contiguously inthe first conductive strip separated by a first slot at a first sideedge of the first conductive strip, the first voltage sense terminal andthe first main terminal formed contiguously in the first conductivestrip; a second voltage sense terminal and a second main terminal formedcontiguously in the second conductive strip separated by a second slotat a second side edge of the second conductive strip, the second voltagesense terminal and the second main terminal formed contiguously in thesecond conductive strip, the first side edge of the first conductivestrip and the second side edge of the second conductive strip formingopposite edges of the resistor; the resistive strip comprising a thirdslot positioned between the first and second voltage sense terminals, adepth of the third slot configured to adjust a TCR value of theresistor; and, the resistive strip comprising a fourth slot positionedbetween the first and second main terminals.
 19. A resistor havingtemperature coefficient of resistance (TCR) compensation, the resistorcomprising: a resistive strip disposed between a first conductive stripand a second conductive strip, the resistive strip having a first sideand an opposite second side, a first end, and an opposite second end;the first conductive strip having an inner edge adjacent the first sideof the resistive element and an outer edge opposite the inner edge, thefirst conductive strip having a first main terminal and a first voltagesense terminal formed contiguously in the first conductive strip andseparated by a first slot formed with a first opening at the outer edgeof the first conductive strip, the first main terminal being larger thanthe first voltage sense terminal; the second conductive strip having aninner edge adjacent the second side of the resistive element and anouter edge opposite the inner edge, the second conductive strip having asecond main terminal and a second voltage sense terminal formedcontiguously in the second conductive strip and separated by a secondslot formed with a second opening at the outer edge of the secondconductive strip, the second main terminal being larger than the secondvoltage sense terminal; wherein the outer edge of the first conductivestrip and the outer edge of the second conductive strip are at differentends of the resistor; a third slot formed with an opening at an edge ofthe first end of the resistive strip between the first and secondvoltage sense terminals; and, a fourth slot formed with an opening at anedge of the second end of the resistive strip between the first andsecond main terminals configured to adjust a resistance value of theresistor.
 20. A resistor with temperature coefficient of resistance(TCR) compensation, the resistor comprising: a resistive strip having afirst edge, second edge, a first side and second side, the resistivestrip disposed between a first conductive strip on the first side of theresistive strip, and a second conductive strip on the second side of theresistive strip; a first main terminal and a first voltage senseterminal formed contiguously in the first conductive strip, the firstvoltage sense terminal positioned adjacent the first edge of theresistive strip; a second main terminal and a second voltage senseterminal formed contiguously in the second conductive strip, the secondvoltage sense terminal positioned adjacent the first edge of theresistive strip; a first slot formed in the conductive strip between thefirst voltage sense terminal and the first main terminal, the first slothaving a first end and a second end, the first end of the first slotpositioned at a side edge of the first conductive strip; a second slotformed in the conductive strip between the second voltage sense terminaland the second main terminal, the second slot having a first end and asecond end, the first end of the second slot positioned at a side edgeof the second conductive strip, wherein the first slot and the secondslot are positioned at opposite edges of the resistor; and, a third slotformed at the first edge of the resistive strip between the first andsecond voltage sense terminals and configured to adjust a TCR value ofthe resistor; a fourth slot formed at the second edge of the resistivestrip between the first and second main terminals and configured toadjust a resistance value of the resistor.